On Oct. 1, when the limit on federally guaranteed loans drops to $625,500 from the current level of $729,750, hundreds of buyers in the city and nearby suburbs will either have to come up with larger down payments to stay under the new limit or face the prospect of applying for jumbo loans — anything above $625,500 — which have higher interest rates. . . . “Across the country this is not a big deal,” said David Maundrell, the president of aptsandlofts.com “but in New York, because our prices are where they are, it’s going to be an issue.”